2015 IBC with commentary
1011.5.2 Riser height and tread depth. Stair riser heights
shall be 7 inches (178 mm) maximum and 4 inches (102 mm)
minimum. The riser height shall be measured vertically
between the nosings of adjacent treads. Rectangular tread
depths shall be 11 inches (279 mm) minimum measured horizontally
between the vertical planes of the foremost projection
of adjacent treads and at a right angle to the tread’s
nosing. Winder treads shall have a minimum tread depth of
11 inches (279 mm) between the vertical planes of the foremost
projection of adjacent treads at the intersections with the
walkline and a minimum tread depth of 10 inches (254 mm)
within the clear width of the stair.
Exceptions:
1. Spiral stairways in accordance with Section
1011.10.
2. Stairways connecting stepped aisles to cross aisles
or concourses shall be permitted to use the riser/
tread dimension in Section 1029.13.2.
3. In Group R-3 occupancies; within dwelling units in
Group R-2 occupancies; and in Group U occupancies
that are accessory to a Group R-3 occupancy or
accessory to individual dwelling units in Group R-2
occupancies; the maximum riser height shall be 73/4
inches (197 mm); the minimum tread depth shall be
10 inches (254 mm); the minimum winder tread
depth at the walkline shall be 10 inches (254 mm);
and the minimum winder tread depth shall be 6
inches (152 mm). A nosing projection not less than
3/4 inch (19.1 mm) but not more than 11/4 inches (32
mm) shall be provided on stairways with solid risers
where the tread depth is less than 11 inches (279
mm).
4. See Section 403.1 of the International Existing
Building Code for the replacement of existing stairways.
5. In Group I-3 facilities, stairways providing access to
guard towers, observation stations and control
rooms, not more than 250 square feet (23 m2) in
area, shall be permitted to have a maximum riser
height of 8 inches (203 mm) and a minimum tread
depth of 9 inches (229 mm).
The riser height—the vertical dimension from tread
surface to tread surface or tread surface to landing
surface—is typically limited to not more than 7 inches
(178 mm) or less than 4 inches (102 mm). The minimum
tread depth—the horizontal distance from the
leading edge (nosing) of one tread to the leading
edge (nosing) of the next adjacent tread or landing—
is typically limited to not less than 11 inches (279
mm) [see Commentary Figure 1011.5.2]. The minimum
tread depth of 11 inches (279 mm) is intended
to accommodate the largest shoe size found in 95
percent of the adult population, allowing for an appropriate
overhang of the foot beyond the tread nosing
while descending a stairway. Tread depths under 11
inches (279 mm) could cause a larger overhang
(depending on the size of the foot) and could force
users with larger feet to increase the angle of their
foot to the line of travel while descending a stairway.
Based on the probability of adequate foot placement,
the rate of misstep with various step sizes and consideration
for the user’s comfort and energy expenditure,
it was agreed that the 11-inch (279 mm)
minimum tread depth and maximum 7-inch (178 mm)
riser height resulted in the reasonable proportion of
riser height and tread depth for stairway construction.
A minimum riser height of 4 inches (102 mm) is considered
to allow the visual identification of the presence
of the riser in ascent or descent.
The precise location of rectangular tread depth and
riser measurements is to be perpendicular to the
tread’s nosing or leading edge. This is to duplicate
the user’s anticipated foot placement in traveling the
stairway.
The size for a winder tread is also considered for
proper foot placement along the walkline [see Commentary
Figure 1011.9 and the commentary for Section
1011.4]. The dimensional requirements are
consistent with the straight tread.
The exceptions apply only to the extent of the text
of each exception. For example, the entire text of
Section 1011.5.2 is set aside for spiral stairways conforming
to Section 1011.10 (see Exception 3). However,
Exception 3 allows a different maximum riser
and minimum tread under limited conditions, but
retains the minimum riser height and measurement
method of Section 1011.5.2.
The requirements for dimensional uniformity are
found in Sections 1011.5.4 and 1011.5.4.1.
Exception 1 is for spiral staircases, a unique type
of stairway. Section 1011.5.2 is not applicable to this
stair type, again because of construction issues and
limited applications. For a discussion on spiral staircases,
see Section 1011.10.
Exception 2 provides a practical exception where
assembly facilities are designed for viewing. See
Sections 1029.13 through 1029.13.2.4 for assembly
stepped aisle-walking surfaces. This exception is limited
to when stairways are a direct continuation of the
path of travel from the level cross aisle to the stepped
aisles. It is not permitted for other stairways within the
assembly space.
Exception 3 allows revisions to the 7 inches/11
inches (178 mm/279 mm) riser/tread requirements for
Group R-3 and any associated utility (such as barns,
connected garages or detached garages) and within
individual units of Group R-2 and their associated utility
areas (such as attached garages). This change is
allowed because of the low occupant load and the
high degree of occupant familiarity with the stairways.
When this exception is taken for stairways that have
solid risers, each tread is required to have a nosing
projection with a minimum dimension of 3/4 inch (19.1
mm) and maximum dimension of 11/4 inches (32 mm)
where the tread depth is less than 11 inches (279
mm). Nosing projections are created where the nosing
of the tread above extends beyond the trailing
edge of the tread below or when a solid riser is
angled under the tread above and connected to the
trailing edge of the tread below. Nosing projections
are not required for residential stairs with open risers
and 10-inch (254 mm) treads. A nosing projection
provides a greater stepping surface for those ascending
the stairway. For users descending the stairway,
the nosing projection allows the toe of the foot to be
placed further away from the riser above, providing
the necessary clearance for the heel of the foot as it
swings down in an arc to its position on the tread (see
Commentary Figure 1011.5.3).
Exception 4 allows for the replacement of an existing
stair. Where a change of occupancy would
require compliance with current standards, this
exception allows a stairway that may be steeper than
that permitted, provided it does not constitute a hazard
[see International Existing Building Code®
(IEBC®) Section 403.1].
Exception 5 allows steeper stairs in spaces of not
more than 250 square feet (23 m2) in correctional
facilities (Group I-3) with a maximum riser height of 8
inches (203 mm) and a minimum tread depth of 9
inches (229 mm) because of the minimal occupant
load and the familiarity of the users with the stairway.
Although not stated in this exception, utilizing a nosing
projection to provide effective tread depth, as
stated in Exception 3 for tread depths less
than 11
inches (279 mm), is a good design practice.
